The present invention relates to an apparatus for welding a web of thermoplastic material using a welding tool which, utilizing the surface of an opposing roller as abutment for the welding tool, forms a welding gap through which the web to be welded is moved toward a conveyor while laying loosely on the opposing roller.
Machines of this type are known from U.S. Pat. No. 2,185,647 and West German Provisional Patent 1,296,356, as well as West German Pat. No. 1,215,350. These known machines serve in particular to produce bags from a continuous two-ply thermoplastic web by simultaneously welding seams and separating individual bags from each other.
These known welding machines, however, have the disadvantage that, when thin webs are welded, the leading edge of the web moving away from the welding gap is disturbed and does not always reach the conveyor for removing the finished bags. To reduce the unsupported distance through which the leading edge of the web must travel past the welding gap the feed rollers which lead the separated sections of web away have been made with the smallest possible diameter so that they can be arranged as close as possible to the welding gap. This solution has the disadvantage that, with the decrease in diameter of these rollers, their flexural strength decreases. When long rollers are used they tend to sag. Experience has shown that, even using take-up rollers of the smallest possible diameter, extremely thin bags still cannot be produced.
In order to remedy this situation, West German Unexamined Application for Patent 24 02 545 proposes providing an abutment for the welding tool which is resiliently supported in an opposing roller and which protrudes by a small amount beyond the outer circumference of the opposing roller. This known opposing roller has chambers therein which are connected by openings with the outer circumference of the opposing roller and within which an elevated pressure or a vacuum can be alternatively produced. In this way, the oncoming web may be drawn against the drum by suction and be repelled from the opposing roller by pressure therein. In these known machines, the web does not lie loosely against the opposing roller but is drawn against the same and is therefore connected with it during the transportation along the surface of the opposing roller.